This invention relates to an ionographic imaging system, and in particular, to an ionographic imaging member having a thick dielectric imaging layer and method of imaging with the thick ionographic imaging member.
In electrography, an electrostatic latent image is formed on a dielectric imaging layer (electroreceptor) by various techniques such as by an ion stream (ionography), stylus, shaped electrode, and the like. Development of the electrostatic latent image may be effected by the application of certain electrostatically charged marking particles.
Ion stream electrographic imaging may be accomplished with the aid of ion projection heads. Movement of the ion stream may be assisted by means of a fluid jet introduced into an ion projection head. For example, fluid jet assisted ion projection heads in electrographic marking apparatus for ion projection printing may utilize ions generated in a chamber, entrained in a rapidly moving fluid stream passing into, through and out of the chamber, modulated in an electroded exit zone by being selectively emitted or inhibited therein, and finally deposited in an imagewise pattern on a relatively movable charge receptor (electroceptor). More specifically, the ion projection head may comprise a source of ionizable, pressurized transport fluid, such as air, and an ion generation housing, having a highly efficient entrainment structure and a modulation structure. Within the ion generation housing there is a corona generator comprising a conductive chamber surrounding a wire, and an entrainment structure which comprises an inlet opening for connecting the source of ionizable fluid into the chamber and for directing the fluid through the corona generator, and an outlet opening for removing ion entraining transport fluid from the chamber. The exiting ion laden transport fluid is directed adjacent to the modulation structure for turning "on" and "off" the ion flow to the charge receptor surface. The chamber, the corona generating source, the inlet opening, the outlet opening and the modulation structure each extends in a direction transverse to the direction of relative movement of the electroceptor. The electroceptor may be uniformly charged by suitable means such as a corona charging device, brush charging, induction charging devices and the like, prior to imagewise discharge of the uniformly charged electroceptor by means of a fluid jet assisted ion projection head. In conventional xerography, corona charging is carried out with a device having a high charge output and a large opening such as a corotron so that a high voltage may be deposited on thick photoconductive insulating layers. A thin electroceptor of less than one half mil having a dielectric constant of about 2 or 3 will not charge up to high electric potentials used in conventional xerography on thick photoconductive insulating layers. Thus, if such an electroceptor is employed in an ordinary ion projection electrographic printing system and is uniformly charged with a device having a high charge output and a large opening such as a corotron, it cannot be charged to high electric potentials. In ionographic systems utilizing fluid jet assisted ion projection heads, only a small amount of ions are emitted due to modulation requirements. Therefore, imagewise discharge of a uniformly charged electroceptor by means of a fluid jet assisted ion projection head results in only a slight change in potential and development density of the electrostatic latent image is poor due to low contrast potential. In U.S. Pat. No. 4,524,371 to N. Sheridon et al, issued Jun. 18, 1985, a fluid jet assisted ion projection printing apparatus is described comprising a housing including ion generating and ion modulating regions. The fluid jet dislodges ions from an electrically biased wire and requires high flow rates to achieve higher deposited charge density. Unfortunately, high fluid flow rates cause a high decibel whistling sound due to the blowers and pumps used to move the fluids. High voltage ion beam deposition is also difficult to achieve when utilizing modulation voltage switching. In addition to the whistling noise problem, it is difficult to obtain more charge out of an ion stream imaging device per unit time. This adversely affects the rotational speed of the electroceptor, i.e. a slower speed electroceptor is needed to achieve a higher charge density. Therefore, one of the drawbacks of ionography is the relatively low charge density and low surface potential which can be supplied to an electroceptor surface while simultaneously attempting to achieve adequate image resolution, print density and throughput speed. Thus, the surface charge potential on the electroceptor in ionographic imaging systems has been considered to be too low for typical dry xerographic development. In other words, although one may form an electrostatic latent image on a thin high dielectric constant electroceptor by means of ordinary ion projection printing systems, the voltage achieved is not high enough for development with a dry, conventional xerographic two-component magnetic brush developer utilizing carrier particles having an electrically insulating outer surface. Thin dielectric imaging layers result in less voltage on the surface and fewer toner particles are pulled from the development system for deposition onto the electroceptor imaging surface. This results in low density toner images due to a combination of low charge density and low voltage. It has, therefore, been generally accepted that high resolution, dense image ionography precludes the use of virtually all the standard dry toner development systems because the achievable development fields (or surface potential) falls below the necessary working range. The underlying reason normally given for this is that the electroceptor has to be very thin or have a low electric field from the image charges in order to accept charge without excessive spreading (blooming) of the deposited charge, yet the electroceptor must be thick enough to provide fields strong enough to drive development. The latter was generally not attainable without also having fields high enough to cause excessive blooming. So the remaining choice was to focus on high charge density and seek a development system which could develop weak fields (e.g. development with liquid ink or single component conductive magnetic brushes containing marking particles having an average particle size of between about 0.1 micrometer and about 15 micrometers). It was believed that the resolution and blooming characteristics were only related to surface charge and field (or surface potential) which were only a function of the dielectric thickness (physical thickness/dielectric constant). For example, in U.S. Pat. No. 4,410,584 to Ando et al issued Aug. 24, 1976, a dielectric imaging member is disclosed having a thickness of about 1 mil (25.4 micrometers). Other patents such as U.S. Pat. No. 4,463,363 to Gundlach et al, U.S. Pat. No. 4,524,371 to Sheridan et al, U.S. Pat. No. 4,644,373 to Sheridan et al, and U.S. Pat. No. 4,584,592 to Tuan et al merely mention a dielectric imaging member but do not appear to provide any dimensions. Some prior art systems have employed low charge modulating ion sources depositing charges of, for example, 17 to 20 nanocoulombs per cm.sup.2. These low charges were too low to be operable with conventional two component development systems utilizing thin, low dielectric constant electroceptors. Further, thin electroceptor or dielectric imaging layer thicknesses are expensive and difficult to process because greater absolute uniformity is necessary to maintain the variance to a small set fraction of the total imaging layer thickness. Thickness variation in an ion stream electrographic imaging system is directly related to the uniformity of the image voltage which is directly related to the developed image quality.
Thus, the prior art ionographic imaging systems utilize low potential charge generating devices, emit an irritating whistling noise at high fluid jet rates and are generally unsuitable for development with standard dry two-component xerographic developers.
Other electrographic systems using dielectric materials such as aluminum oxide materials in the electroceptor exhibit low charge acceptance, high charge decay rates and lateral conduction under ordinary operating conditions. Since aluminum oxide materials are hygroscopic, the electroceptor must be run hot in order to avoid the adverse effects of large variations in ambient humidity [e.g. above 50 percent RH and 23.9.degree. C. (75.degree. F.)] such as image blurring and image retention after erase (ghosting). This electroceptor has too small a dielectric thickness for use in ionographic imaging systems utilizing low potential charge generating devices and standard two component dry xerographic toner development systems.
A stylus, instead of fluid jet ion projection, may be used to charge an electroceptor. Although a stylus is capable of charging dielectric imaging members to high potentials, the stylus itself and/or the imaging member can wear rapidly, produces undesirable fumes and can puncture the electroceptor.
Some prior art xerographic photoreceptors having a thickness of at least about 25 micrometers (1 mil) have been charged to relatively high voltages because of an unlimited power source such as a corotron which are not charge limited. Unfortunately, xerographic photoreceptors require expensive special shipping and storage treatment for protection from temperature extremes or fluctuations, exposure to sun light, contact with reactive fumes and the like. Moreover, special shutter systems, particularly automatic shutter systems, are required in xerographic machines to protect the photoreceptor when it is in use or when it is not in use. Further, photoreceptors are usually sensitive to heat and must be located a safe distance from fusers thereby limiting flexibility in machine architecture design. Also, photoreceptors are sensitive to toner filming. In addition, the coefficient of friction, surface energy and the like of photoreceptors materials, particularly the surface, cannot be readily tailored to accommodate different machine components such as blade cleaning systems. Moreover, cycle up and cycle down problems are a common characteristic of photoreceptors.